The continuing evolution of cardiac troponin I biomarker analysis: from protein to proteoform

Abstract

Introduction: The troponin complex consists of three proteins that fundamentally couple excitation with contraction. Circulating cardiac-specific Troponin I (cTnI) serves as diagnostic biomarker tools for risk stratification of acute coronary syndromes and acute myocardial infarction (MI). Within the heart, cTnI oscillates between inactive and active conformations to either block or disinhibit actinomyosin formation. This molecular mechanism is fine-tuned through extensive protein modifications whose profiles are maladaptively altered with co-morbidities including hypertrophic cardiomyopathy, diabetes, and heart failure. Technological advances in analytical platforms over the last decade enable routine baseline cTnI analysis in patients without cardiovascular complications, and hold potential to expand cTnI readouts that include modified cTnI proteoforms. Areas covered: This review covers the current state, advances, and prospects of analytical platforms that now enable routine baseline cTnI analysis in patients. In parallel, improved mass spectrometry instrumentation and workflows already reveal an array of modified cTnI proteoforms with promising diagnostic implications. Expert commentary: New analytical capabilities provide clinicians and researchers with an opportunity to address important questions surrounding circulating cTnI in the improved diagnosis of specific patient cohorts. These techniques also hold considerable promise for new predictive and prescriptive applications for individualized profiling and improve patient care.

Nanoproteomics enables proteoform-resolved analysis of low-abundance proteins in human serum

“…2f), whereas non-phosphorylated cTnI was found to be the most abundant cTnI proteoform in the dilated cardiomyopathy heart (Fig. 2g), consistent with previous reports suggesting phosphorylation of cTnI as a potential biomarker for heart diseases 27,31,32 . We also identified and characterized C-terminal truncated cTnI proteoforms found in the postmortem samples ( Fig.…”

“…This antibody-free approach can be leveraged in future clinical cTnI diagnostic assays. By further applying to a large human cohort, patient blood samples can be analyzed to comprehensively detect all cTnI proteoforms and establish the relationships between cTnI proteoforms and underlying disease etiology 27 . Ultimately, this nanoproteomics strategy could enable next-generation precision medicine approaches for comprehensive cTnI analysis toward accurate diagnosis, better risk stratification, and improved outcome assessment of patients presenting with various cardiovascular syndromes.…”

“…27). Note that the estimated levels of cTnI released after cardiac injury events such as AMI are typically in the range of 0.5-50 ng/mL 25,27 . Hence, this nanoproteomics approach enables analysis of cTnI proteoforms from human serum at clinically relevant concentrations.…”

“…31). The concurrent enrichment of cTnI and depletion of HSA together with the reliable preservation of the cTnI proteoform fingerprints during the NP enrichment solves the major challenge in the MS detection of cTnI proteoforms directly from the blood 27 and will enable the application of this top-down nanoproteomics strategy to clinical samples to eventually establish a proteoform-resolved cTnI assay.…”

Cardiac troponins may be irreversibly modified by glycation: novel potential mechanisms of cardiac performance modulation

“…Post-translational modification of the troponin subunits is an important mechanism for mediating modulation of cardiomyocyte contractility in health and disease 2 , and novel reports of citrullination 3 , O-GlcNAcylation 4 , 5 , S-nitrosylation 6 , 7 , and phosphorylation 8 – 11 sites on troponins have emerged in recent years. Post-translational modification of cardiac proteins for biomarker evaluation is also gaining interest 12 – 14 . To date, only dynamic post-translational modifications have been investigated, as a mode of reversible signaling regulation of the troponin complex.…”